Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231155
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
555 AM CST Fri Feb 23 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 406 AM CST Fri Feb 23 2018

Primary concern through tonight is the continuing potential for
heavy rain and flooding across the eastern Ozarks into southwest
Illinois tonight.  Secondary concern is a wintry mix of
precipitation in northeast Missouri late tonight.  A weak cold front
is passing through eastern MIssouri at this time.  The front is
expected to stall and become east-west oriented just south of the I-
70 corridor today.  Guidance is showing relatively mild temperatures
mainly in the 50s across most of the forecast area today, and this
looks reasonable given the lows this morning in the low to mid 40s.
Most of the day should be dry as low level flow will be parallel to
the front, but this will change toward the end of the day as 850mb
flow turns to the south-southwest in response to cyclogenesis over
the Plains as a deep upper level trof moves into the eastern
Rockies.  This will result in moderate to strong moisture
convergence in the vicinity of the stationary front.  This is an
excellent set up for a prolonged period of rain...likely beginning
during the early evening across the eastern Ozarks and spreading
northward into central and east central Missouri and southern
Illinois.  While it`s likely that the entire area will see
precipitation tonight, the eastern Ozarks and far southern Illinois
have the highest potential for flooding due to the terrain and the
greater amount of QPF forecast in those areas.  Will therefore keep
the flood watch going with no changes this morning.

Light wintry precip is likely on the northern edge of our forecast
area in northeast Missouri late Friday night.  Temperatures are
marginal for freezing rain/sleet...primarily in Knox county between
about 300 AM and 600 AM Saturday morning.  The surface temperature
falls to freezing or just below, and there is a bit of dry air
trying to push in, so think there`s a chance of some sleet due to
wetbulbing, but the primary precip type is likely to be freezing
rain.  That being said, QPF is very light that far north, and
daytime temperatures on Friday will be well above freezing.  Expect
little or no impact due to this wintry precip at this time.

Carney

.LONG TERM...  (Saturday through Thursday)
Issued at 406 AM CST Fri Feb 23 2018

The latest guidance continues to support an active weather period
continuing on Saturday and Saturday Night. Precipitation is expected
to be rather widespread at 12Z Saturday in response to lift
associated with a broad cyclonically curved LLJ and a migratory
impulse in the southwest flow aloft. Across the far northern part of
the CWA, early morning temps on Saturday will be wavering near
freezing, supporting the potential for some light freezing rain
and light ice accumulations before temps climb decidedly above
freezing by mid-morning. Further south the main zone of heavier
rain/showers will be centered from southeast MO into southwest and
south central IL where lift is focused north of an east-west warm
front. I think there will be a min in precipitation from late
morning into the early afternoon as the impulse translates east
and LLJ veers some. However, showers and thunderstorms will be on
the upswing by mid-afternoon into the evening as the LLJ
intensifies and backs to more southwesterly, and large scale
ascent spreads across the region associated with a short wave trof
ejecting northeast into the area from the central Plains. In
response to the ejecting short wave, a deepening surface low and
trailing cold front advance eastward into central MO by 00Z. The
consensus of the models show the warm front will retreat
northward to around I-70 in response to the motion of the surface
low, but the warm sector could be quite diluted that far north
owing to persistent showers and thunderstorms in the zone across
southeast MO through southwest and south central IL. This southern
zone where moisture/PWs and instability is highest will support
the greatest coverage of showers and thunderstorms, highest
rainfall rates, and training and thus where the heavy rainfall
threat will be maximized within the CWA. This is also the same
area which experienced soaking rains earlier this week resulting
in increased flood potential. The flood watch covering this area
looks on target for the anticipated threat/risk. A few strong-
severe thunderstorms will also be possible within the same
southwest- northeast corridor from mid-afternoon into the evening.
The cold front is forecast to move rather quickly across the
remainder of the CWA on Saturday evening in response to the
ejecting short wave trof, with CAA and drying overnight.

Weak surface high pressure will dominate the area on Sunday while
another weaker short wave traverses the region late Sunday into
Sunday night. Moisture at this time appears quite limited with this
short wave and thus its passage looks dry at this time.  The period
Monday-Wednesday will feature warming temps, back to above normal
levels, as heights rise aloft and southerly flow returns. An
impulse in the southwest flow aloft along with low level moisture
return will bring a chance of rain/showers Tuesday night. Another
more potent short wave trof and attendant surface frontal system
will then impact the area late Wednesday into Thursday bringing
yet another round of showers.

Glass

&&

.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 540 AM CST Fri Feb 23 2018

IFR flight condition...primarily due to low ceilings...will
prevail across central and eastern Missouri as well as west
central and southwest Illinois. There is a large hole in the low
ceilings over east central Missouri and southwest Illinois at this
time...and am honestly not sure exactly how to handle this. Think
the most likely scenario is that IFR ceilings will spread into
this area from the west-northwest this morning, and then there
will be gradual lifting through the day across the region. The
next round of rain will begin moving into our area from the
south-southwest early this evening bringing IFR conditions for
tonight.

SPECIFICS FOR KSTL:

IFR ceilings to the west-northwest of Lambert will likely
overspread the terminal this morning...though have low confidence
in how long the IFR will prevail. Should see improvement in the
ceilings by early afternoon regardless. The next batch of rain
will move into the STL metro area by early to mid evening bringing
another round of IFR ceilings and MVFR to IFR visibility

Carney

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch through Saturday evening for Crawford MO-Iron MO-
     Madison MO-Reynolds MO-Saint Francois MO-Sainte Genevieve
     MO-Washington MO.

IL...Flood Watch through Saturday evening for Randolph IL-Washington
     IL.

&&

$$

WFO LSX



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